Like most photosynthetic microorganisms, the access to this resource consists essentially in the assisted culture in the suitable reactors. Light being their main substrate, the culture medium must have an optical interface receiving a light flux. The difficulty of cultivating photosynthetic microorganisms resides in that they are themselves obstacles to the passage of light which is their main substrate. The culture growth will therefore stabilize when light will no more penetrate the thickness of culture. This phenomenon is called self-shadowing.
The length of the optical path, or “light path length”, varying generally between a few centimeters and a few decimeters, allows to characterize of the different modes of containment, and to determine essentially the biomass production per unit time and unit optical area (surface productivity in g/m2/day) and the culture concentration (g/L) in the final growth phase.
The photosynthesis reaction is also accompanied by a consumption of carbon dioxide (CO2) and a production of oxygen (O2). The excess of oxygen inhibits the reaction, while the absence of carbon dioxide interrupts it by lack of substrate to be transformed. A gas/liquid interface therefore must be arranged for the mass transfers between these gases and the liquid phase. In order to promote these exchanges and avoid the heterogeneities, the culture must hold a mixture intended to renew organisms at the aforementioned optical interface and also at the gas/liquid interface.
It is well known, particularly in GB 2 118 572 A, ES 2 193 860 A1, GB 2 331 762 A, ES 2 150 389 A1, FR 2 685 344 A1 and FR 2 875 511 A3, to use closed photosynthetic reactors comprising a closed loop within which the liquid culture medium circulates, said closed loop comprising a reaction pipe provided with reaction sections made from a material transparent to light radiation, and a closing pipe ensuring the connection between the two opposite ends of the reaction pipe.
The reaction pipe of photobioreactors generally consists of horizontal transparent tubes, made of glass or plastic material, with a thickness or a diameter in the order of a few centimeters, which are end-to-end connected by elbows and collectors to form together a single coil-shaped pipe or parallel back-and-forth pins.
The closing pipe comprises a vertical tube called ascending wherein the liquid medium moves upward, and a vertical tube called descending wherein the liquid medium moves downward in particular due to gravity.
The gas injection system generally implemented in photobioreactors consists of an airlift, otherwise said “gas-lift” or gas lift device, that is to say a gas injection at the base of the ascending vertical tube of the closing pipe; said gas injection being used to both put into circulation the liquid reaction medium and to perform gas-liquid exchanges. The airlift includes in its upper portion a head or an enlarged volume tank wherein lower circulation speeds allow gas-liquid separation, and the descending vertical tube of the closing pipe opens into the bottom of the head tank to feed the reaction pipe with liquid.
The aforementioned photobioreactors apply the principle according to which the reaction takes place only in the liquid phase, in other words these photobioreactors seek to minimize the volume of gas injected into the reactor so as not to decrease accordingly the volume of liquid culture medium, in order not to reduce production. Thus, in these photobioreactors, the extraction of oxygen is often performed by means of an ascending vertical closing tube defined hereinabove; said ascending vertical tube forming a bubble column opening into the head tank receiving the liquid culture medium, and including a gas injection in the lower portion, opportunely, of CO2-enriched air. As described hereinabove, the two functions of circulation and gas transfer are combined within this unique device, called airlift, which creates an ascending vertical circulation by momentum exchange between the liquid mass and the gas bubbles resulting from the injection. The photosynthetic oxygen in supersaturation in the liquid passes into the gas phase by air sweeping, while the CO2 passes into solution. These degassing and carbonatation functions are indispensable and intervene simultaneously and inseparably at this unique device wherein the culture must pass according to a high frequency in order to avoid adeleterious increase in the dissolved oxygen content.
The airlifts have the drawback of generating gas bubbles that move upward within the ascending vertical tube of the photobioreactor closing pipe. Indeed, the Applicant has observed the deleterious role of these bubbles for the cultivation of microorganisms in the photobioreactors:                on one hand, the bubbles mechanically stress the microalgae and may harm fragile microorganisms; and        on the other hand, bubbles capture by surface-active effect the bodies which present the surfactant properties, in particular organic molecules, cellular debris and excretion products of living cells. These substances, usually dispersed in the medium in the absence of bubbles, are thereby assembled in the form of aggregates at the free surface of the head tank when the bubbles burst. Bacteria and fungi that could not develop due to the high dilution of these organic bodies find then concentrated substrates favorable to their development.        
One of the purposes of the present invention is to avoid, or at least limit, the formation of bubbles in order to:                contain the bacterial and fungal development, for example to remain compatible with the health standards conventionally imposed in microorganism cultivation; and to        limit the mechanical stresses in the liquid culture medium, and thus allow the cultivation of certain fragile microorganisms which were so far excluded from such a culture in reactor.        
In an alternative embodiment of the airlift, the deoxygenation of the liquid culture medium flowing in the photobioreactor is achieved by gravitationally dropping the liquid medium in a container to a constant level. The liquid culture medium here is put into circulation by a pumping means, in particular of the centrifugal pump type, disposed in the reaction pipe designed not only to compensate for head losses in the pipe but also to elevate the cultivation of the drop height.
Although it generates fewer bubbles, this device with centrifugal pump is at least as mechanically damaging to microorganisms as the airlift. Indeed, in order to overcome the head losses, there is a generation, at each passage through the pumping means, of mechanical stresses that can hinder the growth of microorganisms and cause mortalities within the culture. The production performances become thus altered, sometimes in an unacceptable manner.
Generally, the shears create tensions that can alter cellular integrity by tearing of the wall of microorganisms and effusion of the cytosol, and the accelerations alter the structure of the cell by an increase in the gravitational field.
In addition, the Applicant has observed that the yield in the culture of photobioreactors equipped with airlifts or centrifugal pump was limited in particular because of mechanical stress imposed on photosynthetic organisms. Indeed, the Applicant has established that these stresses arise largely from the phenomena involved in the gas-liquid transfer in order to improve its efficiency and to avoid the inhibitory effects of high oxygen content. Modeling the gas-liquid transfer of carbon dioxide intended to the reaction and the oxygen that it produces requires the determination of the transfer speed that is characterized by the surface transfer coefficient.
This surface transfer coefficient is equal to the product of the volumetric coefficient of matter transfer to the liquid “KL” (m·s−1) and the interfacial area related to the volume “a” (m−1), and therefore depends on the geometry of the gas/liquid exchange system but also on the physicochemical properties of the liquid and gas. In the case of a gas/liquid exchange within a vertical bubble column, the exchange surface depends on the number of bubbles and on their size. The population of bubbles generated by injection of gas in a liquid depends on the injection flow rate, on the geometry of the injector, and on the pressure difference on either side thereof.